South Korea's president has made a very public demonstration of his role as commander-in-chief of the country's armed forces. In a nationally televised speech to military leaders, he refrained from explicitly blaming North Korea for a deadly ship sinking, but vowed to boost his country's military capabilities.

South Korean President Lee Myung-bak came the closest yet Tuesday to saying that the sinking of a navy warship was a deliberate act.

He says one sure fact is that the Cheonan did not sink because of a simple accident.

The Cheonan, a South Korean warship, was patrolling waters west of the Korean peninsula in March when an explosion ripped it in half and sunk it, killing 46 sailors. North Korea has long disputed the maritime border in the area. The two countries fought the latest of three sea skirmishes there last November.

An investigation is ongoing, and no conclusive evidence has emerged implicating North Korea. Still, speculation here in the South of a North Korean role has been mounting. Investigators say the boat was sunk by an "external explosion," and South Korea's defense minister has said he believes a torpedo strike is the most likely scenario.

President Lee says he is waiting for the investigation to run its course.

He says when investigators reveal the reason for the accident, the world will know, and "firm and definite" action will be taken.

A military reprisal by South Korea is almost universally considered unthinkable, because of the risk that it would escalate into a much deadlier war. However, security experts say appearing to do nothing would only embolden North Korea to act even more provocatively.

Mr. Lee has vowed to seek a diplomatic response to the Cheonan sinking via the United Nations Security Council. He also promised top military commanders Tuesday that the armed forces will be strengthened.

He says we must resolutely reform the military in all aspects, including emergency response and reporting systems, intelligence-gathering and discipline.

Mr. Lee adds, South Korea must reflect whether its past approach to defense was handled along lines of "idealism rather than reality."

The two administrations prior to Mr. Lee sought to build on goodwill generated by a historic North-South summit in 2000, but were later widely criticized for coddling Pyongyang.

"This has been an important object lesson for the South Korean public and political parties as to the nature of the North Korean threat," said David Straub, a regional expert with Stanford University's Shorenstein Center for Asia-Pacific Research..

In Tuesday's speech, President Lee said many South Koreans have forgotten that North Korean artillery is aimed at them just 70 kilometers north of Seoul.